Gas flow arrangement apparatus and to apparatus for removing pollutants from gas streams

ABSTRACT

There is disclosed a gas flow arrangement apparatus comprising a gas entrance ( 18 ) and a gas exit ( 20 ), a first flow path from the gas entrance to the gas exit through a means ( 24 ) for at least partly removing at least one pollutant from a gas flow stream and second flow path from the gas entrance to the gas exit other than through the removing means ( 24 ).

FIELD OF THE INVENTION

The present invention relates to gas flow arrangement apparatus and topollutant removal devices, which may incorporate such gas flowarrangements.

BACKGROUND TO THE INVENTION

Pressure is continuing to grow on vehicle manufacturers to reduce theamount of pollutants, especially particulates in gas streams emittedfrom vehicle exhausts. Attempts have been made to collect particulatesfrom gas streams using electro-static precipitation, but generally thesefail because the performance of the apparatus degrades substantiallyover time so it cannot be used in a practical environment.

The present invention finds particular, but not exclusive, applicationin the field of the removal of pollutants from vehicle exhaust gasstreams. In this technological application, often a filter is used toremove pollutants, especially particulate pollutants. However, asparticulate material is built up in the filter, the porosity of thefilter decreases thus increasing back pressure on the exhaust systemwhich can reduce engine efficiency. Since environmental concerns are theprimary reason for removing pollutants, such a decrease in efficiency,with a resultant increase in pollutants, defeats the object of many suchproposed filtration devices.

One particular problem area is in relation to the particulate materialthat is agglomerated. For instance, in a prior art electro-staticprecipitation apparatus of this type, a central electrode is mountedwithin a circular cylindrical solid-walled tube, whereby particulatesare charged by the electrode and attracted to the solid-walledcontainer. However, once particulates arrive at the tube wall over timethey agglomerate and can eventually be swept out through the vehicleexhaust by the continued flow of exhaust gas flow stream over theagglomerated particulate.

In other prior art devices filters have been proposed to removeparticulates from gas streams. However, in this case over timeparticulate build up in the filters reduces their efficiency and causesback-pressure reducing engine efficiency also.

It is an aim of preferred embodiments of the present invention toobviate or overcome at least one disadvantage of the prior art, whetherreferred to herein or otherwise.

SUMMARY OF THE INVENTION

According to the present invention in a first aspect, there is provideda gas flow arrangement apparatus comprising a gas entrance and a gasexit, a first flow path from the gas entrance to the gas exit through ameans for at least partly removing at least one pollutant from a gasflow stream and second flow path from the gas entrance to the gas exitother than through the removing means.

Suitably, gas passing through the pollutant removing means intersectsthe first gas flow.

Thus pressure differences can be minimised and undue back pressure isavoided. To the extent that gas is blocked from a first it can followthe second flow path avoiding the filter.

Suitably, the first flow path diverges from the second flow pathupstream of the pollutant removing means.

Suitably, the first flow path and the second flow path intersect witheach other downstream of the pollutant removing means. Thus the gas inone flow path is reintroduced into the gas of the other flow path.

Suitably, the first gas flow splits from the second gas flow path at aseparator for diverting pollutant to the pollutant removing means.Suitably, the separator is generally conically shaped with an openingfor one of the gas flow paths therethrough.

Suitably, the first flow path diverges from the second flow path at atube through which gas can pass. Suitably, the tube is a perforatedtube.

The first and second flow paths may be in common for some of theirrespective passages through the arrangement, but they form discrete flowpaths before intersecting downstream of the filter.

Suitably, the arrangement comprises a gas flow tube for the second flowpath, which gas flow tube comprises a slot for the first gas flow pathto join the second gas flow path.

Suitably, the arrangement comprises a first chamber, a second chamberand a third chamber, whereby gas enters into a first chamber, passesinto a second chamber at which the first flow path diverges from thesecond flow path, and whereby gas can flow into the third chamberthrough two openings one of which comprises the pollutant removingmeans, and in which there is an exit for gas from the third chamber.

Suitably, the pollutant removing means comprises a filter.

Suitably, the filter comprises a regenerative filter. Suitably, thefilter is electrically regenerative.

Thus, the arrangement provides a gas flow apparatus.

According to the present invention in a second aspect, there is provideda pollutant removal device for at least partly removing a pollutant froma gas flow, the device comprising a gas flow arrangement apparatusaccording to the first aspect of the invention.

Suitably, the device comprises means for at least partially ionising gasflow. Suitably, the ionising means comprises an electrode forelectrostatic precipitation. Suitably, the electrode is mounted in thesecond chamber. Suitably, the electrode is mounted in the first chamber.

Suitably, the apparatus comprises a tube through which the gas stream atleast partly flows, whereby the tube is at least partly porous to thegas stream.

Suitably, the tube is at least partly about the ionising means.

Suitably, the tube is perforated. Suitably, the tube comprises aplurality of holes therethrough. Suitably, the holes are evenly spaced.Suitably, the holes are evenly sized. Suitably, the perforated region ofthe tube is substantially annular. Suitably, the perforated region ofthe tube extends for a substantial length thereof.

Suitably, the tube comprises at least one slot therethrough. Suitably, aplurality of slots is provided. Suitably, the slots are substantiallyevenly distributed about the tube. Suitably, the at least one slot runslongitudinally along the tube.

Suitably, a major portion of the tube is porous. Alternatively a minorportion of the tube is porous.

Suitably, the tube is circular in cross-section.

Suitably, the tube comprises an inlet and an outlet.

Suitably, the cross-sectional area of the tube decreases along itslength from the input to the output thereof.

Suitably, the tube is at least partly coated with a barrier coating forslowing the discharge time of charged agglomerates.

Suitably, the electrode is mounted at one end thereof only.

Suitably, the tube is located in the first and second gas flow paths.The tube acts to split the gas flows and concentrate at least onepollutant in one flow path for subsequent removal.

Suitably, the apparatus comprises a first expansion tube in fluidcommunication with an apparatus gas inlet. Suitably, the diverting tubeextends from the first expansion tube to a second expansion tube definedby the tube. Suitably, there is a third expansion tube about thediverting tube into which gas can flow through the diverting tube.Suitably, a filter is located between (in respect of gas flow) thesecond and third expansion tubes.

Suitably, the device is arranged whereby at least one pollutant isbiased towards the first flow path (ie a substantial majority of aninput pollutant flows through the first flow path, subject to beingtrapped by the filter).

Suitably, a catalytic converter is provided in the second flow path.

Suitably, the electrode projects from the first chamber in to the secondchamber.

Suitably, the second flow path includes a catalytic converter.

Suitably, the device is for fitting to a vehicle exhaust. Suitably, thedevice is for fitting in place of the silencer of a vehicle exhaust.

According to the present invention in the third aspect, there isprovided an apparatus for removing pollutants from a gas stream, theapparatus comprising means for charging particulates in the gas streamand a tube through which the gas stream at least partly flows, wherebythe tube is at least partly porous to the gas stream and the apparatusadditionally comprises means for collecting at least one pollutant.

Suitably, the tube is at least partly about the charging means.Suitably, the charging means comprises an electrode.

Suitably, the tube is perforated. Suitably, the tube comprises aplurality of holes therethrough. Suitably, the holes are evenly spaced.Suitably, the holes are evenly sized. Suitably, the perforated region ofthe tube is substantially annular. Suitably, the perforated region ofthe tube extends for a substantial length thereof.

Suitably, the tube comprises at least one slot therethrough. Suitably, aplurality of slots is provided. Suitably, the slots are substantiallyevenly distributed about the tube. Suitably, the at least one slot runslongitudinally along the tube.

Suitably, a major portion of the tube is porous. Alternatively a minorportion of the tube is porous.

Suitably, the tube is circular in cross-section. Suitably, the tubecomprises an inlet and an outlet.

Suitably, the cross-sectional area of the tube decreases along itslength from the input to the output thereof.

Suitably, the electrode is mounted at one end thereof only.

Suitably, there is a first gas flow path from an apparatus gas inlet toan apparatus gas outlet and a second gas flow path from the apparatusgas inlet to the apparatus gas outlet. The first and second gas flowpaths may be in common for a part thereof. Suitably, a filter is locatedin the second gas flow path. Suitably, the tube is located in the firstand second gas flow paths. The tube acts to split the gas flows andconcentrate at least one pollutant in one flow path for subsequentremoval.

Suitably, the apparatus comprises a first expansion tube in fluidcommunication with an apparatus gas inlet. Suitably, the diverting tubeextends from the first expansion tube to a second expansion tube definedby the tube. Suitably, there is a third expansion tube about thediverting tube into which gas can flow through the diverting tube.Suitably, a filter is located between (in respect of gas flow) thesecond and third expansion tubes.

Suitably, the filter comprises an electrically regenerative filter.

Suitably, the apparatus is for removing pollutants from an exhaust gasstream, preferably a vehicle exhaust gas stream.

According to the present invention in a fourth aspect, there is provideda combustion generator and an apparatus according to the second or thirdaspects of the invention in which exhaust gas from the generator flowsto an apparatus inlet.

Suitably, the generator is an internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example only,with reference to the drawings that follow; in which:

FIG. 1 is a schematic perspective (partly cut away) illustration of agas flow arrangement apparatus according to an embodiment of the presentinvention.

FIG. 2 is a schematic perspective (partly cut away) illustration of thegas flow arrangement shown in FIG. 1 from a reverse angle.

FIG. 3 is a longitudinal cross-sectional view of the arrangement shownin FIGS. 1 and 2.

FIG. 4 is an enlarged partly cut away and sectional drawing of thefilter shown in FIGS. 1 and 2.

FIG. 5 is a schematic partly cut away illustration of an embodiment of aparticulate filtration device according to the present invention.

FIGS. 6 and 7 are schematic partly cut away illustrations of two furtherembodiments of a device according to the present invention.

FIG. 8 is a schematic longitudinal cross-sectional view of an electrodemount.

FIG. 9 is a schematic partly-sectional elevation of a gas flowarrangement apparatus according to a yet further embodiment of thepresent invention.

FIG. 10 is a perspective view of a second gas flow path tube and filterof FIG. 9.

FIG. 11 is a sectional view of a further electrode mounting arrangement.

FIG. 12 is a plan elevation (external walls cut away) of an apparatusaccording to a further embodiment of the present invention.

FIG. 13 is a side elevation of FIG. 12.

FIG. 14 is a perspective illustration of FIGS. 12 and 13.

FIG. 15 is a plan elevation (external walls cut away) of an apparatusaccording to a yet further embodiment of the present invention.

FIG. 16 is a perspective illustration of FIG. 15.

FIG. 17 is a plan view of a yet further embodiment of the presentinvention.

FIG. 18 is a side elevation of FIG. 17.

FIG. 19 is a sectional, inverted plan view corresponding to FIG. 17.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1-3 of the drawings that follow, there is shown a gasflow arrangement apparatus within a circular cylindrical tubular bodyindicated by dashed line 2. The body 2 is defined internally by wallplates 4, 6, 8 and 10 respectively into a first chamber 12, a secondchamber 14 and a third chamber 16. The body 2 is provided with a gasentry tube 18 and gas exit tube 20. Gas entry tube 18 extends from theexterior wall plate 4 to first chamber 12. That is, gas enters at theentrance of 18 and exits into first chamber 12. Gas exit tube 20 extendsfrom the exterior of wall plate 10 to third chamber 16. Additionally,there is provided a perforated tube 22 extending between first chamber12 and third chamber 16, the perforations opening into second chamber14. The tube 22 is highly perforated whereby in a given annulus there ismore area taken up by holes than by solid. The preferred structure issubstantially constant radially and longitudinally.

A filter 24 for removing pollutants from the gas stream is mounted inthird chamber 16 about an opening 26 between third chamber 16 and secondchamber 14.

The filter 24 is an electrically regenerative filter such as the filteridentified as 3M part number SK-1739.

The filter 24 is shown in more detail in FIG. 4 of the drawings thatfollow. The filter 24 comprises a tubular outer body 28 of a NEXTEL 312filtration mounted on a porous metallic frame 30 which is connected toearth (which may be a floating earth) at one end 32. The other end 34provides an electrical connection 36 (see also FIGS. 1 and 2) to a powersupply 37 (FIG. 5) to achieve heating and regeneration of the filter 24as is known in the art.

An electrode 38 is mounted on wall plate 10 by a ceramic electrode mount39 to project into the hollow interior of perforated tube 22 as shown incross-section in relation to FIG. 4 of the drawings that follow in whichcorresponding reference numerals are used.

In use, pollutant eg particulate carrying gas enters the arrangement at18 and passes into first chamber 12 from which its only route is intoperforated tube 22. In operation the electrode is highly charged tobetween 18 kV-40 kV negative polarity d.c. to ionise or chargeparticulates in the gas stream forcing them through the perforated holesof the tube 22 in to second chamber 14 (under full load the potentialmay be about 10 kV). Additionally, it is believed that the gas becomesat least partly ionised.

The perforated tube 22 opens into third chamber 16 allowing gas to passthrough exit tube 20 to exhaust. Further, gas can flow from secondchamber 14 to third chamber 14 through hole 26 through filter 24. Thusfilter 24 can collect particulate material. The filter 24 isregenerative so that at intervals it is electrically regenerated. Thisneed not be on a regular basis. However, if for any reason the filter 24does not regenerate fully or a heavy loading occurs causing backpressure between filter 24 and second chamber 14, this is compensatedfor because gas can still flow to exit tube 20 through perforated tube22 and third chamber 16. Thus build up of particulates (or otherpollutants) in filter 24 will not cause undue back pressure on theengine providing an exhaust stream to the gas flow arrangement. As aresult, the problem of back pressure encountered in relation to priorart filtration arrangements is overcome by embodiments of the presentinvention and there is provided a geometrically efficient and compactgas flow arrangement.

Thus embodiments of the present invention provide a first gas flow path40 (FIG. 5) from gas entrance 18 to gas exit 20 via first chamber 12,tube 22, third chamber 16 through filter 24 and second chamber 14 and asecond gas flow path 42 (FIG. 4) from gas entrance 18 to gas exit 20 viafirst chamber 12, tube 22 and second chamber 14 which is other thanthrough the filter 24.

Referring to FIG. 6 of the drawings that follow, there is shown anotherembodiment of a gas flow arrangement and pollutant removal deviceaccording to the present invention. The arrangement and device issimilar to that described in relation to FIG. 5 (and similar referencenumerals are used for corresponding integers), except that the first gasflow path 40 through filter 24 is generally straight on, ie the flowpath does not diverge substantially from the path of the tube 22 to thefilter 24 and the second gas flow path 42 follows the more tortuousroute as shown.

To bias the particulate pollutants to follow first gas flow path 40 atFIG. 6, instead of a highly perforated tube 22 (considered over thelength at tube 22) a small area 50 of perforated tube 52 with a lowerhole density is provided. The less perforated tube 52 is not annular, itjust occupies a slot in the tube. As the effect of the corona dischargeelectrode 38 with the floating earth of the tube 52 is to drawparticulates to the side (tube 52) walls where they tend to agglomerate,by providing less open area for the agglomerated particulate to passthrough, it is less likely that particulates will follow the second flowpath 42.

Another difference in the FIG. 6 embodiment is the provision of acatalytic converter 54 in the second flow path 42 for the removal ofhydrocarbons from the gas stream.

FIG. 7 is a yet further embodiment of the present inventionsubstantially similar to the embodiment of FIG. 6, except that fourequally spaced longitudinal slits 60 are provided over a substantialminority of the surface area of tube 62.

Referring to FIG. 8 of the drawings that follow, the electrode mount 39is shown in more detail. The electrode mount 39 is a one piece ceramicconstruction having a longitudinal hole 64 therethrough for theelectrode 38 (not shown in FIG. 8). The electrode projects from distalend 66 and is connected to a power source at end 68. The electrode mount39 is held by a bracket (not shown) about shoulder 70. Protrusions 72 a,72 b and 72 c project from the exterior of electrode mount 39. Theprotrusions 72 are partly hollow, rebated conical shapes that provide atortuous route from the electrode 38 projecting from distal end 66 toearth to reduce leakage.

Referring to FIGS. 9 and 10 of the drawings that follow, there is showna gas flow arrangement apparatus 80 for use in a pollutant removaldevice in which outer walls are not shown for clarity. The apparatus 80comprises an ionising electrode 82 in an electrode mount 83, partlysurrounded by an electrode hood 84. Electrode 82 extends into anelectrode tube 86 which terminates in an outwardly diverging end 88.Spaced from electrode tube 86 is a second gas flow path tube 90 having agenerally conically shaped entrance 92 with a central opening 94. Theopening 94 is substantially inside the diameter of the walls ofelectrode tube 84. Tube 90 terminates in an exit 98. About tube 90 is acatalytic filter 100 for at least partly removing pollutants from a gasstream passing therethrough.

Operation of the embodiment of FIGS. 9 and 10 is similar to that of theembodiments described above. Exhaust gases, carrying pollutants, enterthe apparatus 90 upstream of electrode 82, and pass over hood 84 whichserves to help prevent pollutant build up on electrode 82. The electrode82 is charged to ionise pollutants in the gas flow, which pollutants aretherefore attracted to the walls of electrode tube 86 as they flowdownstream, leaving relatively cleaner gas towards the centre of theflowstream. The conical opening of second gas flow path tube 90 servesto help deflect pollutant into a first gas flow path (indicatedschematically by arrows labelled 102, while the second gas flow path isindicated by arrows labelled 104). The first gas flow path 102 passesthrough filter 100, which removes some pollutants, and rejoins secondgas flow path 104 through a slot 96 in tube 172 downstream to the filter100. The slot 96 is relatively small compared to the surface area oftube 90. The pressure difference either side of slot 96 is believed toencourage now relatively cleaner gas from the first gas flow pathdownstream of filter 100 to rejoin the second gas flow path. Second gasflow path 104 passes through second gas flow path tube 90 carryingrelatively cleaner gas. The rejoined gas streams, pass out of theapparatus at exit 98.

In any of the embodiments a resistive organic barrier coating may beprovided over the inner surface of the tube (22 in FIG. 1) downstream ofthe beginning of the electrode. The barrier coating is preferably oversubstantially all of the inner surface of the tube. The coating isTLHB/02 available from Camcoat Performance Coatings on 127 Hoyle Street,Bewsey Industrial Estate, Warrington, WA5 5LR, United Kingdom. It isbelieved that by reducing the discharge rate of the agglomeratedparticulates along the tube by providing the coating, the particulatesare more likely to stay in the vicinity of the tube.

Referring to FIG. 11 of the drawings that follow, an alternativeelectrode mounting arrangement is shown. Both the electrode mount 83 andelectrode hood 84 are formed from a ceramic high purity aluminamaterial, sold under the trade mark SINTOX FF which is believed to havea dielectric strength of between 30 and 40 kV/mm.

The electrode mount 83 comprises a first ceramic mounting portion 88 anda second ceramic mounting portion 90 mounted in bore 86. The secondceramic mounting portion 90 is of a reduced external diameter comparedwith the first ceramic mounting portion. The electrode mount 83 can beformed from a single ceramic. Thus the electrode mount 83 has a portionof a first diameter and a portion of a lesser diameter towards thedistal end (from which the electrode projects) thereof. The secondportion 90 of second diameter extends a substantial distance beyond hood84 typically at least 30mm.

The hood 84 protects a substantial part of the electrode (mounted incentral bore 86) from the inflow of pollutants containing gas thusminimising the risk of shorting. However, it is believed that at least a30mm length of the electrode needs to project beyond the hood. It isnoted that the gas inlet is not around the electrode but ratheralongside it and can be protected from it by the hood 84.

The electrode mount and hood can be glazed to reduce pitting of thesurface and hence the build up of articulates thereon. The glaze acts asa means for smoothing the surface of the electrode mount.

It is noted that although the maximum exterior diameter of eachgenerally conically shaped protrusion 83 decreases in a downstreamdirection, the minimum internal diameter are substantially the same±10%. This is believed to provide additional burn-off points ifrequired.

The alumina content of hood and mount is typically at least 80%,normally at least 90%, preferably more than 95%, more preferably morethan 97% and most preferably more than 99%.

Referring to FIGS. 12-14 of the drawings that follow, there is shown afurther embodiment of a gas flow arrangement and apparatus for removingpollutants according to the present invention. In the FIGS. 12-14embodiment, exhaust gas enters through an inlet 100 into a perforatedbaffle tube 102 from which all of the entering exhaust gases flow intofirst chamber 104. In chamber 104, electrode mount 106 over asubstantial part of which lies hood 108 mounts an electrode 110 whichprojects into a second chamber 112 defined by field tube 114. Field tube114 includes an opening in its end to an intermediate chamber 116, theonly exit from which is into filter 118. An alternative flow path isprovided via an opening 120 in the wall of field tube 114. The opening120 is provided with an upstanding lip 122 projecting inwardly into thefield tube 114 at at least the upstream portion thereof, but in thisembodiment along the full length thereof. Further, the opening 120comprises a generally V-shaped upstanding leading edge 124 at anupstream end thereof.

Fluid flow path leads from field tube 114 via opening 120 leads to aperforated exit tube 126. Perforations 128 in exit tube 126 permit gaspassing through filter 118 to re-enter the diverted gas flow leading toexit 130.

It is noted that the leading edge 132 of field tube 114 comprises areturned edge that is curved back on itself whereby the exterior edge ofthe leading edge 132 of field tube 114 is configured relative to theelectrode whereby something else lies between it and electrode and/orelectrode mount. In this case, another part of the field tube liesbetween the external edge and both of the electrode mount 106 andelectrode 110.

Upstanding lip 122 and leading edge 124 help to divert particulates awayfrom opening 120 from which it is intended that cleaner gas flows.Together, upstanding lip 122 and leading edge 124 act as means fordiverting particulates away from the opening 120.

The electrode, electrode mount and hood are not shown in FIG. 15.

Referring to FIGS. 15 and 16 of the drawings that follow, there is showna further gas flow arrangement apparatus and apparatus for removingpollutants according to the present invention.

In FIGS. 15 and 16, the apparatus comprises an inlet 150 into whichexhaust gas flows into a baffle chamber 152 having first exit ports 154and second exit ports 156. First exit ports 154 exit to first clamber158. Second exit ports 156 exit into an intermediate chamber 160 havingholes 162 permitting the flow of gas back into first chamber 158. Anelectrode mount 164 (FIG. 15 only), covered for a substantial partthereof by hood 166 (FIG. 15 only), is provided in first chamber 158 formounting of an electrode 168 (FIG. 15 only) within a field tube 170. Atits downstream end, field tube 170 terminates in an outwardly divergingportion 172 adjacent a generally conical portion 174 within which is atube 176 extending to an exit tube 178.

In exit tube 178 is provided an opening 180 prior to the exit 182 oftube 176.

In use, exhaust gas flows in via inlet 150 into field tube 170 via firstchamber 158. Particulates in the field tube are charged by electrode 168and tend towards the walls of field tube 170. Thus the particulates arediverted from the central flow of gas through field tube 170. Thecentral flow of gas enters tube 176 into exit tube 178. Other gasbearing a higher loading of particulates exits towards the periphery offield tube 170 and therefore tends not to enter tube 176. The generallyconical portion 174 acts as a deflector for the particulates encouragingthem not to enter tube 176. The particulate laden gas exiting field tube170 other than through tube 176 enters a second intermediate chamber 184leading to filter 186. Gas exiting filter 186 can only exit theapparatus via opening 180 and into exit tube 178. However the gasexiting filter 186 tends to be at a low velocity compared to the highvelocity gas exiting tube 176. The pressure differential causes the gasin third chamber 188 about filter 186 to be drawn through opening 180into exit tube 178 and hence to outlet 190.

Field tube 170 may include a curved leading edge 192 as described abovein relation to FIGS. 12-14.

FIGS. 17 and 18 show a further embodiment of the present invention. InFIGS. 17 and 18, for clarity the electrode mount and electrode are notshown.

Referring to FIGS. 17 and 18, there is shown a gas inlet into aperforated expansion chamber 202, from which all the input gas flowsinto a first chamber 204 and from there into field tube 206 which leadsto filter 208. Alternatively, through opening 210 in field tube 206 gascan flow to exit tube 212 in which there is a concentrically mountedflow tube 214 and in an exterior wall of which an opening 216 mountedbehind (relative to the gas flow) the exit 218 of tube 214. In exit tube212 a catalytic body 220, acting as a catalytic converter, optionallycan be mounted. In use, gas enters through inlet 200, passes throughexpansion tube 202 into first chamber 204 and then into field tube 206in which particulates in the gas flow are charged. Charged particulatestend towards the side wall of field tube 206 and an upstanding lip maybe provided around 210 to divert particulates therefrom. Particulatesproceeding from field tube 206 to filter 208 are filtered and the gasflow can continue towards exit 222 via holes 216 into exit 212.

Although the first and second gas flow streams are shown separately inthe same tube or area of the apparatus, this is for explanatory purposesonly and it will be appreciated that in these regions the gas flows areintermingled.

It is noted that there may be a plurality of devices, a plurality offilters and/or a plurality of catalytic converters.

Instead of using standard direct current as described above, highfrequency superimposed a.c can be used.

The reduced gas flow through the filter when compared with acorresponding device in which all of the input gas stream flows throughthe filter makes the electrical regeneration of the filter moreefficient because the thermal effect of the gas flow is correspondinglyreduced.

Preferred embodiments of the present invention find particular benefitin the application of pollutant, especially particulate removal fromexhaust gas streams, especially of internal combustion engines. For suchengines the arrangement can be mounted in place of the vehicle silencerto avoid taking up unnecessary space. The device may be upstream ordownstream of a catalytic converter.

The reader's attention is directed to all papers and documents which arefiled concurrently with or previous to this specification in connectionwith this application and which are open to public inspection with thisspecification, and the contents of all such papers and documents areincorporated herein by reference.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extend to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed.

1. A gas flow arrangement apparatus comprising a gas entrance and a gasexit, a first flow path from the gas entrance to the gas exit through ameans for at least partly removing at least one pollutant from a gasflow stream and second flow path from the gas entrance to the gas exitother than through the removing means.
 2. A gas flow arrangementapparatus according to claim 1, in which the gas flow path passingthrough the pollutant removing means intersects the first gas flow.
 3. Agas flow arrangement apparatus according to claim 1, in which the firstflow path diverges from the second flow path upstream of the pollutantremoving means.
 4. A gas flow arrangement apparatus according to claim1, in which the first flow path and the second flow path intersect witheach other downstream of the pollutant removing means.
 5. A gas flowarrangement apparatus according to claim 1, in which the first gas flowsplits from the second gas flow path at a separator for divertingpollutant to the pollutant removing means.
 6. A gas flow arrangementapparatus according to claim 5, in which the separator is generallyconically shaped with an opening for one of the gas flow pathstherethrough.
 7. A gas flow arrangement apparatus according to claim 1,in which the first flow path diverges from the second flow path at atube through which gas can pass.
 8. A gas flow arrangement apparatusaccording to claim 7, in which the tube is a perforated tube.
 9. A gasflow arrangement apparatus according to claim 1, in which thearrangement comprises a gas flow tube for the second flow path, whichgas flow tube comprises a slot for the first gas flow path to join thesecond gas flow path.
 10. A gas flow arrangement apparatus according toclaim 1, in which the arrangement comprises a first chamber, a secondchamber and a third chamber, whereby gas enters into a first chamber,passes into a second chamber at which the first flow path diverges fromthe second flow path, and whereby gas can flow into the third chamberthrough two openings one of which comprises the pollutant removingmeans, and in which there is an exit for gas from the third chamber. 11.A gas flow arrangement apparatus according to claim 1, in which thepollutant removing means comprises a filter.
 12. A gas flow arrangementapparatus according to claim 11, in which the filter comprises aregenerative filter.
 13. A gas flow arrangement apparatus according toclaim 12, in which the filter is electrically regenerative.
 14. Apollutant removal device for at least partly removing a pollutant from agas flow, the device comprising a gas flow arrangement apparatuscomprising a gas entrance and a gas exit, a first flow path from the gasentrance to the gas exit through a means for at least partly removing atleast one pollutant from a gas flow stream and second flow path from thegas entrance to the gas exit other than through the removing means. 15.A pollutant removal device according to claim 14, in which the devicecomprises means for at least partially ionizing gas flow.
 16. Apollutant removal device according to claim 15, in which the ionizingmeans comprises an electrode for electrostatic precipitation.
 17. Apollutant removal device according to claim 16, in which the electrodeis mounted in the second chamber.
 18. A pollutant removal deviceaccording to claim 17, in which the electrode is mounted in the firstchamber.
 19. A pollutant removal device according to claim 14, in whichthe apparatus comprises a tube through which the gas stream at leastpartly flows, whereby the tube is at least partly porous to the gasstream.
 20. A pollutant removal device according to claim 15 in whichthe apparatus comprises a tube through which the gas stream at leastpartly flows, whereby the tube is at least partly porous to the gasstream, and in which the tube is at least partly about the means.
 21. Apollutant removal device according to claim 19, in which the tube isperforated.
 22. A pollutant removal device according to claim 21, inwhich the tube comprises a plurality of holes therethrough.
 23. Apollutant removal device according to claim 22, in which the holes areevenly spaced.
 24. A pollutant removal device according to claim 22, inwhich the holes are evenly sized.
 25. A pollutant removal deviceaccording to claim 21, in which the perforated region of the tube issubstantially annular.
 26. A pollutant removal device according to claim21, in which the perforated region of the tube extends for a substantiallength thereof.
 27. A pollutant removal device according to claim 19, inwhich the tube comprises at least one slot therethrough.
 28. A pollutantremoval device according to claim 27, in which a plurality of slots isprovided.
 29. A pollutant removal device according to claim 28, in whichthe slots are substantially evenly distributed about the tube.
 30. Apollutant removal device according to claim 27, in which the at leastone slot runs longitudinally along the tube.
 31. A pollutant removaldevice according to claim 19, in which the tube is circular incross-section.
 32. A pollutant removal device according to claim 19, inwhich the tube comprises an inlet and an outlet.
 33. A pollutant removaldevice according to claim 32, in which the cross-sectional area of thetube decreases along its length from the input to the output thereof.34. A pollutant removal device according to claim 20, in which the tubeis at least partly coated with a barrier coating for slowing thedischarge time of charged agglomerates.
 35. A pollutant removal deviceaccording to claim 16, in which the electrode is mounted at one endthereof only.
 36. A pollutant removal device according to claim 20, inwhich the tube is located in the first and second gas flow paths.
 37. Apollutant removal device according to claim 14, in which the apparatuscomprises a first expansion tube in fluid communication with anapparatus gas inlet.
 38. A pollutant removal device according to claim37, in which a diverting tube extends from the first expansion tube to asecond expansion tube defined by the tube.
 39. A pollutant removaldevice according to claim 38, in which there is a third expansion tubeabout the diverting tube into which gas can flow through the divertingtube.
 40. A pollutant removal device according to claim 38, in which afilter is located between (in respect of gas flow) the second and thirdexpansion tubes.
 41. A pollutant removal device according to claim 14,in which the device is arranged whereby at least one pollutant is biasedtowards the first flow path.
 42. A pollutant removal device according toclaim 14, in which a catalytic converter is provided in the second flowpath.
 43. A pollutant removal device according to claim 14, in which thedevice is for fitting to a vehicle exhaust.
 44. A pollutant removaldevice according to claim 43, in which the device is for fitting inplace of the silencer of a vehicle exhaust. 45-74. (canceled)
 75. Acombustion generator comprising an apparatus for removing pollutantsfrom a gas stream, the apparatus comprising means for chargingparticulates in the gas stream and a tube through which the gas streamat least partly flows, whereby the tube is at least partly porous to thegas stream and the apparatus additionally comprises means for collectingat least one pollutant, in which exhaust gas from the generator flows toan apparatus inlet.
 76. A combustion generator according to claim 75, inwhich the generator is an internal combustion engine.